1. Field of the Invention
The present invention relates to a projection type display apparatus (transmission type liquid crystal projector, reflection type liquid crystal projector, or the like). In particular, the invention relates to an automatic focusing device of a front projector.
2. Related Background Art
Some proposals have been made hitherto on a focusing method used for a projection optical system in a projection type display apparatus (hereinafter referred to as projector).
Japanese Patent Application Laid-Open No. H09-197249 discloses this kind of projector having a structure in which a focusing operation is performed such that a reference image is projected onto substantially the center of a projection area on a screen surface by projection optical means, and light reflected by the surface is allowed to pass through a slit and received by a line sensor, and then a position of the light incident on the line sensor is detected.
The projector disclosed in Japanese Patent Application Laid-Open No. H09-197249 requires, however, complicated signal processing where a line sensor is used to receive light of a reference image, and its position on the line sensor is determined by calculation through signal processing. Besides, such a projector costs high because the line sensor itself is expensive. Further, even in the case of using an inexpensive sensor such as a silicon photodiode in place of the expensive line sensor, the structure of the projector disclosed in Japanese Patent Application Laid-Open No. H09-197249 necessitates any mechanical mechanism and involves high costs either way.
In addition, as regards operating environments of a projector, the projector is used under ordinary room lighting in many cases. In such cases, the structure of the projector disclosed in Japanese Patent Application Laid-Open No. H09-197249 has the following problem to attain a sufficient focusing accuracy. That is, since the projector disclosed in Japanese Patent Application Laid-Open No. H09-197249 uses a high-sensitivity sensor such as a line sensor, when a screen is lighted up with normal interior illumination, the sensor detects a good amount of ambient light components in addition to light projected by projection means. As a result, a barycentric position of a reference image to be detected varies depending on the amount thereof. This means that the projector should be used with the interior illumination tuned off in order to realize ideal focusing.